The present invention relates to a process or the preparation of polyolefins, especially polyethylenes, the use of metallocene compounds as catalyst components in the production of such polyolefin and the polyolefins obtainable thereby.
Low density polyethylene (LDPE) offers excellent optical properties and can be processed at relatively low temperatures and pressures while maintaining a good melt strength. LDPE has however limited possibilities for downgauging, due to a low draw ratio, and a low stiffness.
Linear-low-density polyethylene (LLDPE) has greatly improved downgauging possibilities and excellent tear and impact properties; its stiffness however remains low and its processability is well below that of LDPE. Also, conventional LLDPE""s optical properties do not match those of LDPE. Optical properties of LLDPE have been improved by using metallocene-catalysed LDPE (mLLDPE) resins; stiffness is however not improved in these products and the processability of these grades is generally worse than that of conventional LLDPE.
Wherever high rigidity is needed, LDPE and LLDPE compositions will require overly thick structures. Especially for LLDPE, where excellent impact and tear properties render its downgauging capability useful, the lack of rigidity may be a main drawback. High rigidity maybe a requirement for the end product, it is very often a necessity for product handling.
Metallocene catalysts are known in the production of linear low density polyethylenes. EP-A-0653445 describes polyethylenes having a density not higher than 0.94 g/cm3 using a high temperature high pressure solution phase process. Another high temperature high pressure process is described in EP-A-0786466 where the production of LLDPE requires temperatures of at least 120xc2x0 C.
Alternative methods of polyethylene production are disclosed in U.S. Pat. No. 5,317,036 which uses an unsupported catalyst in the gas phase and EP-A-0668295, which uses the gas phase or an unsupported catalyst in slurry phase. The metallocene catalysts of EP-A-0668295 are specially-prepared spray dried, filled metallocene catalysts.
These pocesses all require either expensive or specialised catalyst production or relatively high operating costs.
In the production of other polyethylene compositions, it is possible to use reaction systems based on a Ziegler-Natta catalyst or a chromium-based catalyst. These reaction systems require a high concentration of comonomer. This suffers from a drawback in that high concentration of comonomer results in increased solubility of the polyethylene produced in a slurry process. One consequence of the increased solubility of polymer is that there is a high incidence of reactor fouling. Use of a high concentration of comonomer is also costly because of the need to recycle unincorporated comonomer.
It is an aim of the present invention to overcome these disadvantages.
The present invention provides use of a catalyst system comprising a metallocene catalyst component of general formula Rxe2x80x3 (CpRm)(Cpxe2x80x2Rxe2x80x2n) MQ2 supported on an inert support in the slurry phase production of linear low density polyolefin, wherein Cp is a cyclopentadienyl moiety, Cpxe2x80x2 is a substituted or unsubstituted fluorenyl ring; Rxe2x80x3 is a structural bridge imparting stereorigidity to the component; each R is independently hydrogen or hydrocarbyl having 1 to 20 carbon atoms in which 0xe2x89xa6mxe2x89xa64; each Rxe2x80x2 is independently hydrocarbyl having 1 to 20 carbon atoms, in which 0xe2x89xa6nxe2x89xa68; M is a Group IVB transition metal or vanadium; and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; the metallocene having a centroid-M-centroid angle in the range 105xc2x0 to 125xc2x0.
FIG. 1 shows the effect of decreasing the centroid-M-centroid angle in Zr-based metallocenes. The metallocenes of the present invention have a very open structure which permits the facile incorporation of comonomer with larger substituents such as hexene in polyolefin production. In this way, LLDPE with densities around 0.9 or lower may be produced at a commercially acceptable polymerisation temperature in a slurry process. The production of LLDPE with such low densities has hitherto not been possible with Cr-based and closed structure Cent-Zr-Cent ( greater than 125xc2x0) metallocenes in a loop slurry process. Lower comonomer concentrations need be used in the process thereby reducing the likelihood of reactor fouling and avoiding excessive use of expensive comonomer.
Preferably Cp is a substituent cyclopentadienyl in which each R is independently XR*3 in which X is C or Si and each R* is independently H or hydrocarbyl having 1 to 20 carbon atoms. More preferably the cyclopentadienyl is substituted with Ph2CH, Me3C, Me3Si, Me, Me and Me3C,Me and SiMe3, Me and Ph, or Me and CH3xe2x80x94CHxe2x80x94CH3.
Preferably, each Rxe2x80x2 is independently YRxe2x80x2xe2x80x33 in which Y is C or Si and each Rxe2x80x2xe2x80x3 is independently H or hydrocarbyl having 1 to 20 carbon atoms.
The structural bridge Rxe2x80x3 is generally alkylidene having 1 to 20 carbon atoms, a dialkyl germanium or silicon or siloxane, alkyl phosphine or amine, preferably Mexe2x80x94Cxe2x80x94Me, Phxe2x80x94Cxe2x80x94Ph,xe2x80x94CH3xe2x80x94, Etxe2x80x94Cxe2x80x94Et, Mexe2x80x94Sixe2x80x94Me, Phxe2x80x94Sixe2x80x94Ph or Etxe2x80x94Sixe2x80x94Et.
The metal M is preferably Zr or Hf and each Q is preferably Cl.
In order to maximise comonomer incorporation, it is preferred that the centroid-M-centroid angle is no more than 119xc2x0.
In a further aspect, the present invention provides a process for the preparation of a linear low-density polyolefin, which comprises reacting an olefin monomer with hydrogen and an xcex1-olefin comonomer in a slurry in the presence of a catalyst comprising (i) the metallocene catalyst and (ii) an aluminium-or boron-containing cocatalyst, wherein the catalyst is supported on an inert support. The comonomer is preferably hexene, typically present in an amount of from 2 to 10, preferably 2 to 5% by weight of the total reaction mixture.
Suitable aluminium-containing cocatalysts comprise an alumoxane, an alkyl aluminium and/or a Lewis acid.
The alumoxanes usable in the process of the present invention are well known and preferably comprise oligomeric linear and/or cyclic alkyl alumoxanes represented by the formula: 
for oligomeric, linear alumoxanes and 
for oligomeric, cyclic alumoxane,
wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C1-C8 alkyl group and preferably methyl. Generally, in the preparation of alumoxanes from, for example, aluminium trimethyl and water, a mixture of linear and cyclic compounds is obtained.
Suitable boron-containing cocatalysts may comprise a triphenylcarbenium boronate such as tetrakis-pentafluorophenyl-borato-triphenylcarbenium as described in EP-A-0427696, or those of the general formula [Lxe2x80x2xe2x80x94H]+ [B Ar1 Ar2 X3 X4]xe2x88x92 as described in EP-A-0277004 (page 6, line 30 to page 7, line 7).
The catalyst system is employed in a slurry process, which is heterogeneous. In a slurry process it is necessary to immobilise the catalyst. system on an inert support, particularly a porous solid support such as talc, inorganic oxides and resinous support materials such as polyolefin. Preferably, the support material is an inorganic oxide in its finally divided form.
Suitable inorganic oxide materials which are desirably employed in accordance with this invention include Group 2a, 3a, 4a or 4b metal oxides such as silica, alumina and mixtures thereof. Other inorganic oxides that may be employed either alone or in combination with the silica, or alumina are magnesia, titania, zirconia, and the like. Other suitable support materials, however, can be employed, for example, finely divided functionalized polyolefins such. as finely divided polyethylene.
Preferably, the support is a silica having a surface area comprised between 200 and 900 m2/g and a pore volume comprised between 0.5 and 4 ml/g.
The amount of alumoxane and metallocenes usefully employed in the preparation of the solid support catalyst can vary over a wide range. Preferably the aluminium to transition metal mole ratio is in the range between 1:1 and 100:1, preferably in the range 5:1 and 50:1.
The order of addition of the metallocenes and alumoxane to the support material can vary. In accordance with a preferred embodiment of the present invention alumoxane dissolved in a suitable inert hydrocarbon solvent is added to the support material slurried in the same or other suitable hydrocarbon liquid and thereafter a mixture of the metallocene catalyst component is added to the slurry.
Preferred solvents include mineral oils and the various hydrocarbons which are liquid at reaction temperature and which do not react with the individual ingredients. Illustrative examples of the useful solvents include the alkanes such as pentane, iso-pentane, hexane, heptane, octane and nonane; cycloalkanes such as cyclopentane and cyclohexane, and aromatics such as benzene, toluene, ethylbenzene and diethylbenzene.
Preferably the support material is slurried in toluene and the metallocene and alumoxane are dissolved in toluene prior to addition to the support material.
Where the reaction is performed in a slurry using, for example, isobutane, a reaction temperature in the range 70xc2x0 C. to 110xc2x0 C. may be used.
A linear low-density polyethylene is obtainable from the process with a density below 0.93 g/cc and preferably in the range 0.90 to 0.92. The polyethylene preferably has a molecular weight distribution in the range 2 to 4.5, preferably around 3 and more preferably is partially long chain branched so as to facilitate processing.